A novel IL-7- associated growth factors has been identified that appears to selectively stimulate the proliferation, self-replication and differentiation of pre- pro-B-cells, and to 'prime' them to respond to monomeric IL-7. This pre- pro-B-cell growth-stimulating factor (PPBSF) is a covalently-bound 55 kD heterodimer of IL-7 and as yet unidentified; 30 kD co-factor which has been purified with antibodies raised in IL-7 KO mice. The present application will test the hypothesis that PPBSF is displayed on the plasma membrane of ECM of stromal cells in the subendosteal region of the BM; that it regulated the development of anchorage-dependent pre-pro-B-cells by preferentially binding to low affinity IL-7R; and that it induces the expression of high-affinity IL-7R on their anchorage-independent descendants. The Specific Aims are to: 1) identify and characterize the PPBSF-coF; 2) determine if PPBSF exists in a bound form; 3) define the functions of PPBSF on pre-pro-B-cell and pro-B-cell development in vitro; 4) confirm the existence and functions of PPBSF in vivo; and 5) determine if PPBSF enhances the efficiency of B-lineage engraftment after BM transplantation. Active lines of Ad5.SVR4 transformed BM stromal cells with IL-7 KO mice will be used for large-scale production of PPBSF-coF; and mAbs will be generated by standard hybridoma technology. The identity of the PPBSF-coF will be established by amino acid sequencing of affinity- purified protein. If PPBSF-coF is unique, specific mRNA will be isolated and cDNA probes will be generated for cloning studies and transfection into expression vectors for rPPBSF-coF production. Two dimensional electrophoresis will determine if PPBSF exists as an heterodimer on BM stromal cells and/or in the ECM. The ability of purified PPBSF to induce proliferation, phenotypic differentiation, IgH gene D-J and V-D-J rearrangement, and/or high affinity LL-7R on purified pre-pro-B-cells will be evaluated in vitro. The presence and functions of PPBSF in vivo will be demonstrated by: 1) in situ immunostaining of BM stromal cells; 2) mRNA analysis of BM extracts; 3) alterations in B-cell development after infusion of PPBSF or antibodies to PPBSF-coF; and 4) enhanced efficiency of engraftment of B-lineage cells. These studies not only will provide essential information about the regulation of early B-cell development, but may have implications for improved management for BM transplantation, acute lymphoblastic leukemia, and selected immunodeficiency disorders in human patients.